Fishing Down Marine Food Webs

The large, long-lived fishes at or near the top of aquatic (especially marine)
food webs, when exploited by
multispecies fisheries,
tend to decline faster than smaller, short-lived fishes with lower
trophic levels. This results in the size and mean trophic level of exploited fish assemblages gradually declining, as does
the mean trophic level of catches from an ecosystem exploited in this manner.

This phenomenon, now known as ‘Fishing Down Marine
Food Webs’ (Christensen, 1996;
Pauly et al., 1998), has been documented through
detailed analyses of fisheries catch data from a wide range of ecosystems all over
the world (see
The Science of Fishing Down). The widespread occurrence of ‘fishing down’ is
the reason why, in 2004, the Convention of Biological Diversity (CBD) chose the
mean trophic level of fisheries catches as an index of the biodiversity of large
fishes (defined as fish with trophic levels > 3.5), called the Marine Trophic
Index (see Pauly and Watson, 2005).

This website is designed to encourage further research on and documentation of ‘fishing down’, notably on its intensity (in trophic level units per decade)
and the various
masking factors,
which in certain circumstances, can make its detection difficult.

For accessing the large number of regional, national
and sub-national studies illustrating the occurrence of ‘fishing down’, see
Case Studies. We welcome receiving further case studies (scientific publications or technical reports) documenting 'fishing down'.

For a discussion of perceived objections to ‘fishing
down’, based mainly on imputations and misunderstandings, see
The Nature of the Discord.

These are fisheries in which the non-selective nature of the gear used (e.g., longlines,
trawls, etc.) implies either (1) a targeted catch consisting of a number of species,
or (2) a large by-catch, in addition to the catch of the target species. In the
latter case, a part or all of the by-catch may be discarded.

The number of steps, in a food web, that separates organisms from the primary producers
at the base of that food web. Trophic levels (TL) tend to range in marine food webs
between 1 (phytoplankton, other algae, detritus) and 5 (killer whales, polar bears),
and usually are non-integers (e.g., 2.2 for krill; 3.6 for a mackerel; 4.1 for a
tuna, etc.), because most consumers have an eclectic diet
(see also FishBase ECOLOGY Table ).

Established in 1999, the Sea Around Us Project is a scientific collaboration between The University of British Columbia and The Pew Charitable Trusts.